1. Field of the invention
The invention concerns a device for measuring the ocular parameters of a subject, in particular the interpupillary distance, for the purpose of fitting a pair of spectacles.
2. Description of the prior art
For a pair of spectacles to suit the subject who wears them, it is necessary for the lenses to be disposed in the frame surrounds so that their principal optical axes are coincident with the optical axes of the corresponding eyes, under average conditions of use. The frame constitutes a secondary point of reference for the position of the lenses, its position relative to the face of the subject being essentially defined by the nose pads bearing on opposite sides of the nose and, as an ancillary to this, by the contact of the frame side members with the ears.
The position of the lenses relative to the frame is conveniently defined (in Cartesian coordinates with an axis of abscissae which is horizontal (with reference to the face of the subject) and a vertical axis of ordinates) by the abscissa and the ordinate of the optical center of the lens. The origin of the abscissae is naturally situated in the vertical plane of symmetry of the frame. For reasons of convenience with regard to the finishing of the lenses, there is generally taken as the origin of the ordinates the horizontal straight line which passes through the bottom part of the frame surrounds, so that the ordinate is the distance between the optical center of the lens and its bottom edge.
The interpupillary distance is the distance which separates the optical axes of the eyes when the gaze is focused at infinity. The (vector) difference between the abscissae of the optical centers of the lenses must correspond to the subject's interpupillary distance.
French patent No. 1 506 352, filed Aug. 4 1966, describes an improved device for measuring the interpupillary distance by determining for each eye the position of the reflection on the cornea of a light spot situated at infinity, with the gaze focused at infinity. Similar measurements are obtained with the gaze focused on a point at a finite distance, in which case the optical axes of the eyes converge.
According to the aforementioned patent, the device essentially comprises a box in which there are placed a lens adapted to be moved parallel to itself along its principal optical axis, a light source at the focus of the lens when the latter is at an origin farthest removed from the source, the latter being geometrically offset from the optical axis by means of a semi-reflecting mirror disposed at 45.degree., and on two respective opposite sides of the box perpendicular to the optical axis of the lens, a hole situated on the optical axis at the focus of the lens when at the origin, and a front plate comprising nose pads and two openings to simulate a spectacle frame, the openings surrounding the intended position of the lenses and being fitted with movable markers constituting a graticule. For purposes of measurement the movable markers and the reflections of the point source on the corneas, as seen through the hole, are rendered coincident. The position of the lens defines for the subject an effective viewing distance from the point source, whereas the operator looks through the hole which is at a point which is optically coincident at the point source at all times. Measurement of the interpupillary distance for a convergent gaze focused on a close virtual point does not introduce any angular parallax error, and the position of the graticule in the plane of the windows, which corresponds to the general plane of the lenses, defines the required position for the optical center of the lenses.
This device, which is of excellent design from the optical point of view, suffers from imperfections in the mechanical part controlling displacement of the graticules. The required accuracy of a few tenths of a millimeter imposes the use of micrometer screws or racks to move the indexes, resulting in a fragile device which must be operated slowly. Reading the position with verniers or drums associated with linear scales becomes difficult as soon as the accuracy required is greater than one-half-millimeter. Also, movement of the graticules in two mutually perpendicular directions seriously complicates the mechanical control and display devices. In point of fact, the device described in the aforementioned patent has only horizontally movable indexes, principally for determining the interpupillary distance, and as an ancillary to this for measuring the distance from the spectacle frame to the eye, by viewing from the side.
An object of the invention is a device for measuring optical parameters capable of rapidly providing accurate data for fitting lenses in a spectacle frame to the ocular parameters of a subject.
Another object of the invention is a device of this kind which provides, in addition to the distance between the pupils and between each pupil and the axis of symmetry of the frame, the distance between the pupil and the bottom edge of the frame.
A further object of the invention is a device of this kind which produces directly usable digital data.